1. Field of the Invention
The present invention is directed to a semiconductor device, and more particularly, to a semiconductor device having an electrostatic discharge (ESD) protective circuit utilizing a CMOS buffer formed around an input/output pad.
2. Discussion of the Related Art
Since the introduction of 16M bit DRAMs, CMOS buffers have been widely used as input/output buffers. The CMOS buffer also functions as an electrostatic discharge (ESD) protective circuit. The structure and operational theory of a conventional CMOS buffer as an electrostatic discharge protective circuit is described below.
FIG. 1 illustrates the layout of an electrostatic discharge protective circuit of a conventional semiconductor device, namely, the CMOS buffer. FIG. 2 is a cross-sectional view of the CMOS buffer taken along line II-II' in FIG. 1.
In FIGS. 1 and 2, an input/output pad 2 is formed on a p-type semiconductor substrate 1. A PMOS transistor including a gate electrode 31, a source 32 and a drain 33 is formed at a left side of the input/output pad 2. The source 32 and the drain 33 are formed by implanting a p-type impurity into the semiconductor substrate 1. The source 32 and the drain 33 are connected to a Vcc terminal 5 and to the input/output pad 2, respectively. Around the source 32 and drain 33 of the PMOS transistor, n+ regions 34, 35 (Vcc diffusion layers) are formed in the semiconductor substrate, separated by an interval "a" of a field region and connected to the Vcc terminal 5. An NMOS transistor, including a gate electrode 41, a source 42 and a drain 43, is formed at a right side of the pad 2. The source 42 and the drain 43 are formed by implanting an n-type impurity into the p-type semiconductor substrate 1. The source 42 and the drain 43 are connected to a Vss terminal 6 and the input/output pad 2, respectively. Around the source 42 and drain 43 of the NMOS transistor, n+ regions 44, 45 (Vss diffusion layers) are formed, separated by an interval "a" of the field region and connected to the Vss terminal 6. An n-well 3 and a p-well 4 (not shown in FIG. 1) are formed in the p-type semiconductor substrate 1. The NMOS transistor and the PMOS transistor are formed in a p-well 4 and a n-well 3, respectively.
In the n-well 3, where the PMOS transistor is formed, a horizontal parasitic PNP bipolar transistor Q2 is also formed utilizing the source 32 of the PMOS transistor, the n-well 3 and the drain 33 of the PMOS transistor for its collector 32, base 3 and emitter 33. A vertical parasitic PNP bipolar transistor Q1 is also formed utilizing the emitter 33 of the horizontal PNP transistor Q2, the n-well 3 and the p-type semiconductor substrate 1 as an emitter 33 a base 3 and a collector 1. The emitter 33 is connected in series with the horizontal PNP transistor Q2. In addition, in the n-well 3, the Vcc diffusion layers 34, 35 are formed at the right and left sides of the PNP transistor Q2, separated by the interval "a" of the field region, and a base resistance Rn1 of a vertical PNP transistor Q1 is connected between a diode formed by the emitter 33 and the base 3 of the vertical PNP transistor Q1 and the Vcc diffusion layer 35. The Vcc diffusion layers 34, 35 are connected to the Vcc terminal 5.
In the p-well 4, an NMOS transistor including the gate electrode 41, the source 42 and the drain 43 is formed. A horizontal parasitic NPN bipolar transistor Q3 utilizes the source 42 of the NMOS transistor, the p-well 4 and the drain 43 as an emitter 42, a base 4 and a collector 43. The Vss diffusion layers 44, 45 are formed at right and left sides of the NPN transistor Q3 and connected to the Vss terminal 6.
The operation of the above-described CMOS buffer as an electrostatic discharge protective circuit will now be explained.
First, static electricity at the pad 2 is discharged through the following discharge path under a negative stress condition, i.e., when the static electricity has a voltage lower than Vss.
A negative charge inputted to the pad 2 is transferred to the n+ region 43 of the horizontal NPN transistor Q3. As the negative charge is transferred to the n+ region 43, a forward bias is formed between the n+ region 43 and the p-well 4, and electrons (minority carriers) move into the p-well 4. Then, the voltage of the p-well 4 is lowered due to the electrons transferred from the n+ region, and a forward bias is formed on the p-well 4 and the n+ region 42. Therefore, electrons move to the Vss terminal having a relatively high voltage through the n+ region 42, and are discharged.
When a voltage lower than Vss is applied to the pad 2, the voltage is discharged through a relatively simple path. The discharge path is formed by a forward biased junction, and the semiconductor (active) elements are therefore not damaged.
The discharge path for a positive charge condition, that is, when a positive voltage (compared to Vss and Vcc) is applied to the input/output pad 2, is relatively complicated. A reverse bias is formed between the n+ region 43 of the horizontal NPN transistor Q3 and the p-well 4, which can result in active element destruction. The discharge path for the case where a positive voltage is applied to the input/output pad 2 will now be described.
A first discharge path is formed through the base resistance Rn1, which is connected in series to the forward diode formed by the emitter 33 and base 3 of the vertical PNP transistor Q1. This discharge path is efficient for the positive voltage when current is low.
A second discharge path is formed by the NPN transistor Q3. This discharge path is used when a high electric current is conducted. First, the pad voltage is passed through the forward emitter-base diode of the vertical PNP transistor Q1 and the base resistance Rn1, and then the high voltage is applied to the n+ region 43 of the horizontal NPN transistor Q3, and forms a reverse biased diode between the n+ region 43 and the p-well 4. When the voltage of the input/output pad 2 rises constantly and exceeds an avalanche breakdown voltage of the diode formed between the n+ region 43 and the p-well 4, the horizontal NPN transistor Q3 is turned on, and thus a high electric current flows. Consequently, the positive charge being transferred through the n+ region 43 is discharged through the n+ region 42 to the Vss terminal 6, which is at a low voltage. However, if application of the high voltage continues, a second breakdown of the semiconductor elements can occur, and this can result in active (semiconductor) element destruction.
A third discharge path is represented by the current flow through the vertical PNP transistor Q1 to the substrate 1. When the semiconductor substrate 1 is connected to Vss, the positive charge is discharged to Vss. In the other cases, the semiconductor substrate current is increased, and thereby the horizontal NPN transistor Q3 is turned on.
The conventional electrostatic discharge protective circuit has a disadvantage with the second discharge path formed by the horizontal NPN transistor Q3. That is, the discharge of a positive charge is mostly through the emitter-base diode of the vertical PNP transistor Q1 and the base resistance Rn1. However, when the base resistance Rn1 is large, the positive charge cannot be efficiently discharged through the emitter-base diode of the vertical PNP transistor Q1 and the base resistance Rn1, and thus the remaining charge must be discharged through a different path, namely, the horizontal NPN transistor Q3. Here, the rising pad voltage is passed through the emitter-base diode of the vertical PNP transistor Q1 and the base resistance Rn1, and the higher input/output pad voltage causes the reverse biasing of the junction of the n+ region 43 of the NPN transistor Q3 and the p-well 4. When the pad voltage exceeds the avalanche breakdown voltage, the horizontal NPN transistor Q3 is turned on and the positive charge is discharged. However, during the high voltage condition, when the pad voltage exceeds the avalanche breakdown voltage, a second breakdown can occur, resulting in the active element destruction.